Staged catalytic cracking reaction systems have been introduced to improve the overall gasoline yields and octane quality of gasoline. In recent times, however, environmental constraints have also had a large impact on the refiner. As a result, the known staged catalytic cracking processes are not sufficiently effective in concomitantly meeting environmental constraints and maintaining a high quality octane gasoline product.
U.S. Pat. No. 5,152,883 discloses a fluid catalytic cracking unit that includes two catalytic cracking reaction steps in series. After a hydrocarbon feed is cracked in a first catalytic cracking reaction step, light hydrocarbon gases and gasoline products are removed from the product stream and the heavier product portion is hydrotreated. Following hydrotreating and further gasoline product removal, the heavier hydrotreated product is cracked in a second catalytic cracking step. The gasoline products are removed and the heavier products are recycled into the hydrotreating process.
Rehbein et al., Paper 8 from Fifth World Petroleum Progress, Jun. 1-5, 1959, Fifth World Petroleum Congress, Inc., N.Y., pages 103-122 (which corresponds to U.S. Pat. No. 2,956,003, Marshall et al.), disclose a two stage catalytic cracking process which uses a short contact time riser as the first stage. The first stage is described as being designed to give 40-50 wt. % conversion. As set forth in the reference, the second stage is a dense bed system that uses gas oils from the first stage along with a recycle stream to give overall conversions of 63-72 wt. %, even though the unit is operated at low enough charge rates to achieve total conversions from 65-90 wt. %.
As set forth above, known catalytic cracking processes which have been integrated with hydrotreating processes are effective in significantly increasing gasoline yield and octane. However, this octane increase is obtained by sacrificing the quality of mid-distillates, which can be used as diesel or heating oil. Moreover, such processes undesirably produce a relatively high quantity of light saturated vapor products resulting from the detrimental hydrogen transfer from the heavier cracked products back to lighter olefin products. By minimizing the negative effects of this type of hydrogen transfer, a greater quantity of olefins product could be produced, and these olefins could be made available for further conversion into oxygenates and useful polymer materials.
The products of conventional FCC processes are generally low in hydrogen content resulting from both the relatively low feed hydrogen content and conventional FCC operating conditions of high temperature, (i.e., above 850.degree. F.) and low pressure (i.e., below about 100 psig). The conventional processes consequently favor the formation of olefinic and aromatic products rather than aliphatic, or hydrogen-rich products. As recent environmental and regulatory pressures have resulted in requirements of higher hydrogen content fuels, especially in the diesel boiling range, a need for hydrogenation of FCC feedstocks and products has also grown. Moreover, there is a need for fuels having a diminished concentration of sulfur-containing species and, the value of FCC units as producers of olefinic gases for chemical feedstocks, e.g., propylene and ethylene, has grown. Hydrogenation technology can be employed to provide enrichment of the hydrogen content of FCC feeds. However, this hydrogen addition must be done wisely in order to maximize utilization of the hydrogen that is consumed and to minimize investment required for the hydrogenation step, while making the best use of FCC equipment as well. It is, therefore, desirable to obtain a combined staged catalytic cracking staged hydroprocessing process which maximizes olefins production, distillate quality and octane level.